In microlithography, critical dimension (CD) linearity error may be of greater importance than, for example, other light beam characteristics, such as, optical resolution. CD linearity error is a measurement of the amount an actual CD of a printed image deviates from an expected or target CD.
The CD linearity errors for a conventional optical imaging system may be dependent on an optical beam waist and/or side lobes in the focal spot. The side lobes may also affect corners and other 2D features. Thus, reduction in CD linearity errors may improve image fidelity.
A proper balance between the optical beam waist and the intensity of side lobes may be dependent on a CD linearity error threshold and may be achieved by optical beam apodization.
Apodization (e.g., optimization of the pupil function), is well-known in optics, but is conventionally used to reduce (e.g., minimize) the width of the central lobe (e.g., beam waist). However, for example, if the central lobe is reduced in excess, CD linearity may be affected by the intensity of side lobes, which may cause the stray light appearance in a printed image and/or may result in an increase in CD linearity error.